Heat transfer management is a concern for designers of light-emitting diodes (LEDs). When LEDs are driven with high currents, high device temperatures may occur because of insufficient heat transfer from the p-n junction of the semiconductor die to the ambient environment. Such high temperatures can harm the semiconductor and lead to such degradations as accelerated aging, separation of the LED chip from the lead frame, and breakage of bond wires.
In addition to the aforementioned problems, the optical properties of the LED vary with temperature, as well. As an example, the light output of an LED typically decreases with increased junction temperature. Also, the emitted wavelength can change with temperature due to a change in the semiconductor bandgap energy.
The main path for heat dissipation (thermal path) in prior art is from the p-n junction to the lead frame and then through the ends of the leads via heat conduction. At the ends of the leads, heat conduction, convection and radiation serve to transfer heat away from the LED when mounted on a printed circuit board (PCB). There is also a secondary path of heat conduction from the surface of the semiconductor die to the surface of the plastic casing. The problem with this design is that the majority of the lead frame sits within the plastic casing, which acts as a thermal insulator, and the main path for heat dissipation out of the device is limited by the size of the leads. Even designs that have added to the size or number of leads in an effort to promote heat transfer still possess an inherent bottleneck for heat dissipation, as the leads are still sandwiched in the thermally insulative plastic casing.